Ah,this might be due to the system beingset to ME Wake in S3, S4-S5, occasionally that causes scripting problems because it takes time to wake the ME before it can process messages. On an AMT 5 system, there are two options. The better of the two is to bump the idle timeout up, if your MEBx is the same as mine this is under Intel AMT Configuration -> Idle Timeout (if you set it to the maximum of 65535, the ME will stay on for about 45 days). You could also set the Power Control so that the ME was On in all power states, instead of On in S0, ME Wake in S3, S4-S5, but this power package was removed in AMT 6.
Also,for future reference it looks like the the Dell download page for the Optiplex 960 is here: http://support.dell.com/support/downloads/driverslist.aspx?c=us&cs=555&l=en&s=biz&ServiceTag=&System...= (at least currently, it might change over time of course), theAMT OS packages arein the Chipset Section. There's one that says it applies to AMT HECI, and the other that says it applies to SOL/LMS, both together are the OS level AMT packages. Just power control shouldn't need either package, but some of the other features might need these packages to be installed.